Hi Ti82,
You can define the batch number automatically by using the auto batch number functionality. You need to create a new batch group, for automatic batches.
Inventory Management> Setup> Dimensions

Later attach on the product you want to receive as auto batch,
Released Products> Manage Inventory Tab> Tracking> Batch number group

The batch number would appear automatically through this way on your PO header.
Please mark YES against the answer that helps you. Thanks
Warm Regards,
Zain